1. Field of the Invention
The present invention relates to gas turbines and more particularly to gas turbines in which combustion is carried out therein.
2. Description of the Prior Art
At present, conventional gas turbines are used for generation of electric power since the equipment is simple, low cost, and can be started up quickly. However, fuel efficiency has been low--about 25% versus 35% for alternatives using steam systems--and with the current high price of fuel, gas turbines have not found application except for short time use at peak load where efficiency is not an overriding factor. Turbines also require a reasonably clean fuel, which is no longer available at low cost in the form of natural gas. Even so, gas turbines are used extensively to fill the demand for electric power during periods of peak demand, as for several hours during the evening. If efficiency could be improved significantly, turbines would become attractive for "base load" operation--that is, for continuous operation.
Turbine efficiency can be increased by raising the maximum operating temperature, although equipment costs increase sharply due to the need for special materials of construction. At present, inlet temperatures of 2000.degree. to 2200.degree. F. on gas turbines are used, and allowable temperature is expected to increase in the future, to perhaps 3000.degree. F. within the next decade or two. Conventionally, hot gas is supplied to the turbine from a precombustor burning a fuel with air to supply hot gas to the turbine at maximum allowable temperature, as limited by mechanical construction. This combustor is at elevated pressure and normally uses a clean gas or liquid fuel. Gas temperature and pressure both decrease considerably during expansion through the turbine, depending mainly on the ratio of inlet to outlet pressure, such that the outlet gas may be at perhaps 1000.degree. F., for example. Its residual heat content is then recovered by heat exchange, typically by exchange with the cool compressed gas or air going to the combustor. This heat exchanger is commonly called a recuperator, and can increase the efficiency of a turbine system considerably; however, its range of application is limited by the practical economic design temperature at which it can operate, for example, 1000.degree. to 1200.degree. F. maximum. As a result, efficiency for the usual turbine systems leaves considerable room for improvement.
Similarly, maximum turbine inlet temperatures have a practical mechanical limit today of about 2000.degree. to 2200.degree. F., and the potentially high efficiency that is theoretically possible has not been achieved. Great effort is directed at ways to allow higher operating temperature, as by cooling the blades using air or water, or by improved materials, and ceramic blades or coatings. Operating pressure could be increased within the range of current technology, but it has not allowed a marked increase in efficiency with present systems.
A variety of modifications to gas turbines have been proposed to increase work output. More particularly, U.S. Pat. Nos. 2,238,905 and 2,478,851 propose reheating of the gas between turbines. The latter patent also proposes the use of conventional external combustors between turbine stages to heat a portion of the gases between stages to high temperatures. However, such a process can bring in excessively hot gases onto the turbine blades and thereby cause mechanical problems.